The present invention relates to a shadow RAM (Random Access Memory) cell using a ferroelectric capacitor, a non-volatile memory device and a controlling method therefor. More specifically, the present invention relates to a shadow RAM in which each memory cell is constituted of an unloaded four-transistor type SRAM (Static RAM) cell added with ferroelectric capacitors and in which a high speed reading/writing operation is carried out by use of the unloaded four-transistor type SRAM when an electric power is being supplied and a non-volatile memory is realized by use of the ferroelectric capacitors when no electric power is supplied.
Heretofore, various shadow RAMs constituted of a combination of the ferroelectric capacitors and the SRAM cell have been proposed. When an electric power is being supplied, these shadow RAMs can store information in the SRAM cell, and can carry out the reading/writing operation of a high speed comparable to that obtained in a conventional SRAM. In a store operation before the electric power is shut off, the information stored in the SRAM cell is transferred into a polarization direction in the ferroelectric capacitor, so that when no electric power is supplied, a non-volatile memory is realized. In other words, the shadow RAM using the ferroelectric capacitor is a memory device having two features, the non-volatile feature of the ferroelectric capacitor and the high speed operation of the SRAM.
For example, a shadow RAM using a ferroelectric capacitor, as disclosed in Japanese Patent Application No. Heisei 11-099534 (published as JP-A-2000-293989), has a memory cell structure as shown in FIG. 2. A flipflop 3 is constituted by connecting an output of each of two inverters 1 and 2 to an input of the other inverter. Two storage nodes Q0 and Q1 of the flipflop are connected to an inverted bit line BLN and a non-inverted bit line BLT through NMOS transistors M0 and M1 acting as an access transistor, respectively. The inverted bit line BLN and the non-inverted bit line BLT are paired, and are connected at their end to a sense amplifier (not shown) for comparing respective voltages on the inverted and non-inverted bit lines to each other.
Furthermore, the bit lines are connected to a writing circuit (not shown) for selectively connecting either of the pair of bit lines to a ground potential in a writing operation, and a precharge circuit (not shown) for precharging the bit lines to either a power supply voltage or the ground potential. Respective gate electrodes of the NMOS transistors M0 and M1 are connected to a common word line WL. The word line WL is connected to a decoder circuit (not shown) for selectively driving one word line to be accessed in accordance with an address signal. Ferroelectric capacitors F0 and F1 having their one end connected to a common plate line PL are connected to the storage nodes Q0 and Q1, respectively. The plate line PL is connected to a plate line driving circuit for maintaining the plate line at Vcc/2 in a power supply condition excluding a store operation and a recall operation.
Now, an operation of the prior art shadow RAM using the ferroelectric capacitors will be described. Incidentally, it would be apparent that the information writing and reading operations for the flipflop 3 is similar to those in the conventional SRAM in the prior art. In an idling operation accompanied with neither a reading nor a writing, all of the word lines are pulled down, the bit lines are precharged to an appropriate potential, and the writing circuit is stopped, so that information in the flipflop is retained.
In order to write information into the flipflop 3, a selected word line WL is pulled up by action of the address decoder, and simultaneously, the writing circuit is driven to bring one of the pair of bit lines BLT and BLN to a low level in accordance with data to be written. When the word line is pulled up, the MOS transistors M0 and M1 are turned on. Since the writing circuit has a driving capability sufficiently larger than that of the inverters 1 and 2, the storage node connected through the MOS transistor to the bit line pulled down by the writing circuit, will be pulled down to the ground potential. Simultaneously, the other storage node is pulled up to the power supply voltage, so that the flipflop becomes stabilized.
In order to read the data from the flipflop 3, after the pair of bit lines are precharged to a high level, a word line is selected, and a potential difference appearing between the selected pair of bit lines is amplified by the sense amplifier. When the word line is pulled up, the MOS transistor interconnecting between the storage node of the low level and the bit line is turned on, so that the bit line concerned starts to drop. The other bit line maintains the high level, since the MOS transistor is not turned on. By discriminating the potential difference between the pair of bit lines by action of the sense amplifier, the information stored in the flipflop is read out.
Now, a store operation will be described with reference to FIG. 3 and FIG. 4. FIG. 3 illustrates a hysteresis on a Q-V plane of the ferroelectric capacitors F0 and F1, and FIG. 4 is a timing chart illustrating waveforms on various parts in the store operation. When the electric power is shut off, the data stored in the flipflop is transferred to the polarization direction of the ferroelectric capacitors F0 and F1. This operation is called xe2x80x9cstorexe2x80x9d. This store operation is triggered by a drop of the power supply voltage or a store signal supplied before the shutoff of the electric power. The store operation will be carried out in the following procedures:
First, when the store operation starts, the plate line PL is at Vcc/2, so that in accordance with the data stored in the flipflop 3, xe2x88x92Vcc/2 is applied across one of the ferroelectric capacitors connected to the storage node of 0 V, while Vcc/2 is applied across the other ferroelectric capacitor connected to the storage node of the power supply voltage (Vcc).
Here, voltages Vc0 and Vc1 applied across the ferroelectric capacitors F0 and F1, respectively, are defined to be a potential difference between one terminal connected to the plate line PL and the other terminal connected to the storage node Q0 or Q1, by considering the potential at the one terminal connected to the plate line PL as a reference.
Next, the plate line PL is pulled up to Vcc. At this time, in the ferroelectric capacitor previously applied with Vcc/2, the voltages at opposite ends of that ferroelectric capacitor become Vcc, with the result that the voltage across the ferroelectric capacitor becomes 0 V. xe2x88x92Vcc is applied across the other ferroelectric capacitor, so that the condition of the other ferroelectric capacitor moves to a point C in the hysteresis loop shown in FIG. 3.
Thereafter, the plate line PL is pulled down to 0 V. At this time, Vcc is applied across the ferroelectric capacitor connected to the storage node of Vcc, so that the ferroelectric capacitor moves to a point A in the hysteresis loop shown in FIG. 3. Simultaneously, the ferroelectric capacitor that was at the point C moves to a point D, so that a negative remanent polarization is held.
Finally, the electric power is shut off. After the shutoff of the electric power, each node is converged to the ground potential. Accordingly, the ferroelectric capacitor that was at the point A finally moves to a point B, so that a positive remanent polarization is held. Since the ferroelectric capacitor can hold the remanent polarization not less than ten years in a condition in which no voltage is applied, the conventional shadow RAM using the ferroelectric capacitor can realize a non-volatile memory.
Next, the recall operation will be described with timing charts of various parts shown in FIG. 5. When the electric power is turned on, the data held in the ferroelectric capacitors is transferred to the flipflop. This operation is called xe2x80x9crecallxe2x80x9d. When the electric power is turned on, if only the power supply voltage of the flipflop 3 is pulled up while maintaining the word line WL and the plate line PL at the low level, it becomes possible to reproduce in the flipflop 3 the data stored as the remanent polarization in the ferroelectric capacitors. When the power supply voltage of the flipflop 3 starts to rise up, the potential of the storage nodes correspondingly rises up by action of the coupling of MOS transistors forming the inverters 1 and 2. Thus, the potential applied across each ferroelectric capacitor increases from 0 V towards a positive large voltage.
The ferroelectric capacitor holding the positive remanent polarization as the point B in FIG. 3 behaves as a small capacitance in comparison with that of the ferroelectric capacitor holding the negative remanent polarization as the point D. This would be apparent from the fact that the locus from the point B to the point A (the arrow Y1) depicted on the Q-V plane has an inclination gentler than that of the locus from the point D to the point A (the arrow Y2). Namely, the storage node connected to the former capacitance (smaller capacitance) rises up quicker than the other node.
If the power supply voltage continues to rise up so that one of the storage nodes exceeds a threshold voltage of the transistors constituting the inverters 1 and 2, a positive feedback is applied to the flipflop 3 so that a voltage difference between the storage nodes is amplified. Ultimately, the storage node held at the point B is brought into Vcc, and the storage node held at the point D is brought into the ground voltage. Finally, the plate line is set to Vcc/2, so that the ferroelectric capacitors are put in an idle condition. As a result, the ferroelectric capacitor which held Vcc before the shutoff of the electric power, holds the data at the point B, and then holds Vcc again after the electric power is turned on again. Similarly, the ferroelectric capacitor which held 0 V before the shutoff of the electric power, holds the data at the point D, and then, holds 0 V again after the electric power is turned on again.
As mentioned above, in the conventional shadow RAM using the ferroelectric capacitor, the data stored in the flipflop 3 is preserved after the electric power is shut off and then turned on. Therefore, it operates as a non-volatile memory. In addition, the data reading and the data writing can be performed similar to the conventional SRAM, since the flipflop 3 and the MOS transistors M0 and M1 function similar to the conventional SRAM.
It is known to constitute a memory cell disclosed in JP-A-63-201998, as a non-volatile memory device using a ferroelectric capacitor, by using one transistor and one ferroelectric capacitor, or alternatively two transistors and two ferroelectric capacitors. This stores information by the polarization direction of the ferroelectric capacitor, regardless of whether or not the electric power is supplied. In addition, since the reading becomes a destructive reading, the writing is carried out following the reading. Therefore, the number of accesses to the ferroelectric capacitor becomes very large, and under the present fabricating technology, reliability after use over a long time is not necessarily satisfactorily ensured. In the shadow RAM using the ferroelectric capacitor, on the other hand, the access to the ferroelectric capacitor is carried out only in the storing operation and in the recall operation, and therefore, a satisfactory reliability of the product can be ensured even in a ferroelectric capacitor having a relative poor performance.
In the prior art shadow RAM using the ferroelectric capacitor, the memory cell is constituted of one SRAM memory cell composed of the flipflop and the access transistors, and the two ferroelectric capacitors. Therefore, in the case that the flipflop is formed of a CMOS circuit, the number of elements in the SRAM memory cell part becomes six (six transistors). As mentioned above, in the prior art shadow RAM, the memory cell for storing one bit of information is constituted of the six transistors and the two ferroelectric capacitors, the increase in the sell size is unavoidable in comparison with the non-volatile memory device as disclosed in JP-A-63-201998, and therefore, it has a problem which is not suitable to increase the storage capacity.
Accordingly, it is an object of the present invention to provide a shadow RAM cell using a ferroelectric capacitor, in which the number of transistors constituting a memory cell is reduced so that the storage capacity can be increased, a non-volatile memory device and a control method therefor.
According to the present invention, there is provided a memory cell characterized by comprising a pair of drive transistors cross-connected each other by connecting a gate of each drive transistor to a drain of the other drive transistor at a pair of storage nodes, a pair of access transistors on-off controlled by a common word line for selectively connecting the pair of storage nodes to a pair of bit lines, respectively, and a pair of ferroelectric capacitors having one end thereof directly connected to the pair of storage nodes, respectively, and the other end thereof connected to a plate line.
Furthermore, it is characterized in that an off-leak current of the access transistors is not less than ten times an off-leak current of the drive transistors. It is also characterized in that a precharge potential of the bit lines is a power supply voltage, so that at the storage node of a high potential side, both the drive transistor and the access transistor are off, and the storage node of the high potential side is held at a high potential near to the power supply voltage by action of the off-leak current of the access transistor, and on the other hand, at the storage potential of a low potential side, the drive transistor is in an on condition and the storage potential of the low potential side holds the ground potential.
Furthermore, it is characterized in that, in a store operation, the plate line is swung between the ground potential and the power supply potential while maintaining the pair of access transistors in an off condition, and in a recall operation, the potential of the bit line is pulled up from the ground potential to a predetermined power supply potential while maintaining the pair of access transistors in the off condition and while maintaining the plate line at the ground potential.
In addition, it is characterized in that, in the recall operation, the word line is brought to a second predetermined potential which is between the ground potential and the power supply potential, so that the pair of access transistors are put in an on condition of some degree, with the result that the storage node of the high potential side is more quickly pulled up to the power supply potential.
Moreover, it is characterized in that, in a power supplying condition excluding the store operation and the recall operation, the potential of the plate line is set to a first predetermined potential which is between the ground potential and the power supply potential, and this first predetermined potential is about a half of the power supply potential. It is also characterized in that, in the store operation, the potential of the plate line is changed from the first predetermined potential to the power supply potential and then to the ground potential in the order, or alternatively to the ground potential and then to the power supply potential in the order.
It is characterized in that, when the plate line is pulled down from the power supply potential or the first predetermined potential to the ground potential, this voltage pulling-down is carried slowly or in a step-by-step manner to spend a time of not less than 0.1 millisecond, for the purpose of ensuring that the potential of the storage node of the high potential side does not lower and the drive transistor of the low potential side is never turned off.
Alternatively, it is characterized in that, when the plate line is pulled down from the power supply potential or the first predetermined potential to the ground potential, the potential of the word line is brought to the second predetermined potential which is between the ground potential and the power supply potential, and the pair of access transistors are turned on to some degree, for the purpose of ensuring that the potential of the storage node of the high potential side does not lower and the drive transistor of the low potential side is never turned off.
Furthermore, it is characterized in that, in a power supplying condition excluding the store operation and the recall operation, a potential difference between opposite ends of each of the pair of ferroelectric capacitors (first predetermined potentialxe2x80x94ground potential) and (power supply potentialxe2x80x94first predetermined potential) is smaller than a coercive voltage of the ferroelectric capacitor.
According to the present invention, there is also provided a nonvolatile semiconductor memory device characterized by locating the memory cell mentioned above at each intersection of the pair of bit lines and the word line to form a matrix. It is also characterized in that the plate line of all the memory cells is connected in common. It is characterized by further including a plate line drive circuit for driving the common-connected plate line.
Furthermore, it is characterized in that, in the store operation, the plate line drive circuit drives the common-connected plate line from the first predetermined potential which is between the ground potential and the power supply potential, to the power supply potential and then to the ground potential in the order, or alternatively to the ground potential and then to the power supply potential in the order, and in that the plate line drive circuit carries out the store operation for all the memory cells connected to the plate line, in one lump. It is also characterized in that when the common-connected plate line is pulled down from the power supply potential or the first predetermined potential to the ground potential, the plate line drive circuit pulls down the voltage slowly or in a step-by-step manner to spend a time of not less than 0.1 millisecond.
In addition, it is characterized in that, in a power supplying condition excluding the store operation and the recall operation, the plate line drive circuit sets the potential of the common-connected plate line to a first predetermined potential which is between the ground potential and the power supply potential. It is also characterized in that, in the power supplying condition excluding the store operation and the recall operation, a potential difference between opposite ends of each of the pair of ferroelectric capacitors (first predetermined potentialxe2x80x94ground potential) and (power supply potentialxe2x80x94first predetermined potential) is set to be smaller than a coercive voltage of the ferroelectric capacitor.
According to the present invention, there is further provided a method for controlling a memory cell which comprises a pair of drive transistors cross-connected each other by connecting a gate of each drive transistor to a drain of the other drive transistor at a pair of storage nodes, a pair of access transistors on-off controlled by a common word line for selectively connecting the pair of storage nodes to a pair of bit lines, respectively, and a pair of ferroelectric capacitors having one end thereof directly connected to the pair of storage nodes, respectively, and the other end thereof connected to a plate line, characterized in that in a store operation, the plate line is swung between the ground potential and the power supply potential while maintaining the pair of access transistors in an off condition.
In addition, it is characterized in that, in a recall operation, the potential of the bit line is pulled up from the ground potential to a predetermined power supply potential while maintaining the pair of switching elements in the off condition and while maintaining the plate line at the ground potential, and in a power supplying condition excluding the store operation and the recall operation, the potential of the plate line is set to a first predetermined potential which is between the ground potential and the power supply potential.
Furthermore, it is characterized in that, in the store operation, the potential of the plate line is changed from the first predetermined potential to the power supply potential and then to the ground potential in the order, or alternatively to the ground potential and then to the power supply potential in the order, and in that when the plate line is pulled down from the power supply potential or the first predetermined potential to the ground potential, the voltage is pulled down slowly or in a step-by-step manner to spend a time of not less than 0.1 millisecond.
It is also characterized in that, in a power supplying operation excluding the store operation and the recall operation, a potential difference between opposite ends of each of the pair of ferroelectric capacitors (first predetermined potentialxe2x80x94ground potential) and (power supply potentialxe2x80x94first predetermined potential) is smaller than a coercive voltage of the ferroelectric capacitor.
Now, an operation of the present invention will be described. In the shadow RAM of the present invention, the memory cell is constructed by directly connecting the ferroelectric capacitors to the storage nodes of an unloaded 4-transistor SRAM cell. Therefore, the shadow RAM of the present invention can be constructed of four transistors which are less than those required in the prior art shadow RAM by two transistors, and can realize a storage capacity which is larger than that realized in the prior art shadow RAM. On the other hand, the store operation and the recall operation, which are carried out in the shadow RAM of the present invention, are different from those in the prior art.
In the shadow RAM of the present invention, in the power supplying condition excluding the store operation and the recall operation, the reading and the writing are carried out similarly to those in the prior art unloaded 4-transistor SRAM cell. In the store operation, the plate line is pulled up to the power supply voltage once and then pulled down to 0 V, so that a suitable writing pulse is applied to the ferroelectric capacitor. Incidentally, in order to prevent destruction of the data when the potential of the plate line is pulled down, the potential of the plate line is pulled down slowly or in a step-by-step manner to spend a time of not less than 0.1 millisecond. Alternatively, by turning on the access transistor to some degree, the potential of the storage node of the high potential side is prevented from lowering so that the stored data is protected.
Similarly to the prior art, in the recall operation, the potential of the bit line is simply elevated while maintaining the plate line at the ground potential, so that a voltage is applied across the ferroelectric capacitor, and the change of the capacitance characteristics attributable to the difference in the direction of polarization is amplified by the memory cell and the data is restored. Alternatively, even in the recall operation, the access transistor is put in an on condition of some degree, so that the amplification is performed quickly. Thus, a highly reliable non-volatile memory can be realized by expending a time for the store operation.
In addition, in the shadow RAM of the present invention, the recall operation can be carried out without using a sense amplifier and a write circuit. Therefore, the recall operation can be performed simultaneously in all the memory cells, so that the time required for the recall operation of all the memory cells can be shortened, and a control circuit can be simplified. Furthermore, since the ferroelectric capacitors are connected directly to the storage nodes which indicate either the ground potential or the power supply voltage, it is possible to apply a write pulse of the power supply voltage to the ferroelectric capacitor with no high voltage circuit.
Moreover, in the shadow RAM of the present invention, in the power supplying condition excluding the store operation and the recall operation, the plate line is set to an intermediate potential which is between the power supply potential and the ground potential, so that the voltage applied across the ferroelectric capacitor is relaxed to xc2xd of the power supply potential, with the result that deterioration of the capacitor can be prevented.
As mentioned above, the shadow RAM using the ferroelectric capacitor in accordance with the present invention uses the shadow RAM cell which comprises a pair of drive transistors cross-connected each other by connecting a gate of each drive transistor to a drain of the other drive transistor at a pair of storage nodes, a pair of access transistors on-off controlled by a common word line for selectively connecting the pair of storage nodes to a pair of bit lines, respectively, and a pair of ferroelectric capacitors having one end thereof directly connected to the pair of storage nodes, respectively, and the other end thereof connected to a plate line. Therefore, the shadow RAM using the ferroelectric capacitor in accordance with the present invention can reduce a chip area in comparison with the prior art shadow RAM. On the other hand, similarly to the prior art shadow RAM, since the recall operation and the store operation can be carried out without using a sense amplifier and a write circuit, all the memory cells can be recalled and stored simultaneously. Thus, a control circuit for controlling these operations can be realized with a small size similar to a control circuit of the prior art, since it is unnecessary to repeat these operations.
Furthermore, when data is written into the ferroelectric capacitor, it is possible to apply a write pulse of the power supply potential to the ferroelectric capacitor with using neither a voltage higher than the power supply potential nor a negative voltage. Therefore, in comparison with the prior art shadow RAM configured to apply the power supply voltage to the ferroelectric capacitor by using either a high voltage or a negative voltage, it is possible to expect reduction of the circuit scale, reduction of the chip area, and a process compatibility with a logic circuit which is provided mixedly in one chip and which does not use a high voltage.